Dental porcelains are specific formulations of a class of ceramics called porcelains that were produced in China and other countries by 1500 A.D.
In Philadelphia in 1844, Samuel Stockton White began the manufacture of porcelain teeth for dentures. These high quality teeth were refined during the next 125 years and are still made in large quantities. During that time other uses for dental porcelain were developed requiring modification of the basic composition containing the minerals feldspar, quartz and kaolin. Potash and soda are added for special properties. These denture tooth porcelains were high fusing, 2375.degree.-2550.degree. F. (1300.degree.-1400.degree. C.). Single tooth crowns were also made from porcelain with little or no kaolin resulting in a lower fusing temperature and greater translucency.
Dental porcelains used for coating metals are feldspar glasses resulting from compositions of about 55% silica, 12% aluminum oxide, 14% potash and soda and various oxides for color and translucency control. As result of the composition, these porcelains all contain crystalline lucite particles. Lucite is the high thermal expansion component, 21 PPM/.degree.C. (parts per million per degree Celsius). The low thermal expansion, 9 PPM/.degree.C., is the glassy component, which when combined with the lucite provides the proper thermal expansion for the porcelains to fuse to metal alloys. These porcelains have been fused to metals that have thermal coefficients of expansion that are similar to the porcelains of the proper composition. Examples of such metals are gold-platinum-palladium, palladium-silver, cobalt-chromium, nickel-chromium and others. Failure to properly match the metal and porcelain with regard to the thermal expansion will result in cracking and spalling of the porcelain coating.
Porcelains used for PFM applications fuse between 1650.degree.-1850.degree. F. (900.degree.-1010.degree. C.) which are considered low fusing porcelains. These porcelains are capable of being colored and tinted to match the appearance of natural teeth. They are relatively insoluble in mouth fluids and remain aesthetic for many years.
Although enamels, glasses and ceramic coatings had been used in many prior applications, the concept of "matched thermal coefficients of expansion" for specific porcelains and metals for dental crowns must be credited to the Weinsteins.
Weinstein et al., U.S. Pat. Nos. 3,052,982 and 3,052,983, demonstrate that by special formulations of Si).sub.2 Al.sub.2 O.sub.3 and potassium and/or sodium oxides a porcelain can be obtained that is thermally compatible with certain dental gold alloys to make prosthetic teeth. This means that the thermal coefficients of contraction of the metal and porcelain must be sufficiently similar to prevent spalling and fracture of the coating during cooling. A second requirement is that the molten glassy porcelain must wet the metal and in particular its oxide covered surface. The wetting is necessary but not sufficient. When the porcelain and metal cool to room temperature the porcelain must strongly adhere to the metal. Much research over the last 30 years has dealt with all of these requirements that must be met by the porcelain and alloy to be successful. Gold alloys were the metal of choice at the time. Since then many nonprecious metal alloys have been used to replace gold alloy.
Porcelain fused to metal (PFM) dental applications have the desirable properties of both materials; metal provides strength and toughness and porcelain provides beauty and aesthetics. The PFM technology is among the major developments in modern dentistry.
Nickel-chromium alloy for prosthodontic applications has become, over the last ten years, the major alloy in that field. During this time numerous other alloys have also been used for PFM applications. These alloys include cobalt-chromium, iron-chromium, and combinations of these and nickel-chromium alloys with molybdenum and beryllium. Many alloys containing gold, platinum palladium and silver have been used since 1962.
There has been no known attempt to use these alloys for dental brackets. The stainless steel alloy currently in use in orthodontics for manufacturing brackets has been traditionally accepted. However, stainless steel is not an alloy that meets all of the requirements of a PFM application.